Electrochemical gas sensors operate on fuel cell principles, in that the gas or vapour to be sensed is caused to react at an electrode of an electrochemical cell, thereby generating a current which is a function of the concentration of the gas or vapour to be sensed.
Electrochemical gas sensors are widely used for the detection of toxic gases in a variety of applications. These include measurements in industrial environments for personal protection; combustion emission monitoring from fixed and mobile sources to minimise environmental impact and maximise fuel efficiency; monitoring discharges from fixed plant for regulatory certification; and medical applications. Electrochemical gas sensors based on liquid electrolytes and gas diffusion electrodes, typically employing gas diffusion barriers, have been very successful in addressing these and other applications, as described by B. S. Hobbs et al, “Techniques And Mechanisms In Gas Sensing”, Chapter 6, 1991, Edited by P. T. Moseley et al. These sensors generally provide a robust, accurate, low cost solution to the demands of instrument manufacturers and end users.
However, there is continuing demand for improved levels of performance to enable more accurate and reliable measurements without a significant increase in the cost or complexity of the technology. Improvements can also allow new applications to be addressed where conventional methods are deemed unacceptable. Such demands typically centre on (i) the specificity of the sensor output to the defined measurand of interest; (ii) higher sensitivity to the gas of interest allowing reliable resolution of concentrations well below statutory limits; and (iii) the ability of the sensor to operate to its specified performance across a wide range of environmental conditions (eg. temperature, humidity). The present invention is particularly concerned with reductions in the humidity cross-interference response of such devices, although the principles employed may be used to enhance other aspects of the behaviour.
The humidity of the sensed environment can affect the performance of an electrochemical gas sensor in two main ways. Over the long term, high or low humidity (particularly if combined with extremes of temperature) can significantly alter the properties of the aqueous electrolyte by exchange of water occurring through the gas access to the sensor. If the electrolyte loses water, its pH rises or falls, depending on whether an alkali or acid system is employed. This may result in damage to sensor components and places more strain on the seals retaining electrolyte within the housing. There is also a significant risk of the sensor drying out, resulting in a loss of electrolytic contact between the electrodes and hence a malfunction. If the electrolyte takes on water it becomes more neutral, which may compromise the activity of the system towards the reactions of interest. In extreme cases, leakage of electrolyte due to pressure build-up may occur. The mechanisms governing these behaviours are generally understood, and sensor designers have evolved a number of strategies to address the problems which arise.
In the short term, relatively rapid, or transient, changes in ambient conditions (typically on a timescale of a few seconds) may affect the instantaneous output of the sensor rather than its long term performance. Such changes may occur in industrial applications when using portable gas detection equipment. For example, it is common for such devices to be carried between adjacent areas of plant experiencing differing conditions. In severe cases, reading errors may temporarily produce a fail-to-danger condition by offsetting the true response to a target measurand (although this is usually very short-lived). A more common problem is that the instrument is put into an alarm condition by the very rapid change in output it experiences. False alarms are very undesirable since they may lead to unwarranted interruptions in operation of the plant and cause a loss of confidence in the instrument.
The mechanisms governing such transient effects are not well understood, but are believed to relate to changes occurring at the 3-phase interface between the gas, liquid electrolyte and solid catalyst which governs the behaviour of these electrochemical systems. The severity of such effects is more marked in some electrochemical systems than others and can be particularly troublesome in sensors with inherently low sensitivity, since in such cases a comparatively small shift in output current is interpreted as a relatively high gas concentration. For example, sensors for ppm-level detection of chlorine and hydrogen chloride employing graphite catalysts are particularly prone to humidity transient problems.
Another situation in which rapid humidity changes are inevitably encountered is in the real-time analysis of exhaled breath. Electrochemical sensors have been used in medical applications such as smoking cessation for many years. However, successful applications have usually involved the measurement of relatively high concentrations (ppm levels or above) of comparatively straightforward measurands such as CO, where high activity can be achieved by conventional sensor designs. Other potential applications are more challenging. One of these is the use of nitric oxide (NO) concentrations as an indicator of pulmonary function and a means of early identification of asthma or other related problems. See, for example, A. D. Smith et al, N. Engl. J. Med. (2005); 352:2163-73; K. Ashutosh, Curr. Opin. Pulm. Med. (2000); 6:21-5; and F. L. Ricciardolo et al, Physiol. Rev. (2004); 84:731-65.
Electrochemical gas sensors utilising conventional graphite electrocatalysts as the basis of their sensing electrodes are capable of detecting NO at the medically relevant concentrations of 0-200 ppb under favourable, controlled conditions. However, they can also exhibit transient signals of several ppm NO equivalent in response to a 0-100% step change in relative humidity (RH). This represents a severe difficulty when the typical level of NO in exhaled breath for a healthy subject is ˜15 ppb, but is present in a background whose humidity content can reach 100% RH and varies rapidly with time. A further concern is the additional presence in exhaled breath of other potential cross-interferents, such as CO2 (˜5%), NH3, H2, CH4, ketones, ethanol etc., at variable trace levels.
Historically, such gas sensors have been used in industrial safety applications where humidity effects, whilst undesirable, did not necessarily represent a critical problem, bearing in mind the 8-hour exposure limit of 25 ppm NO. However, new evidence has significantly increased concern about the toxicity of NO and the latest recommendation is that exposure levels should be kept below 1 ppm. Therefore, the requirement to address the problem of humidity cross-interference whether in the form of a steady-state (ie. long term) interference or a transient interference is driven not only by the desire to address new applications, but also by the need to respond to more stringent demands from existing users.
One potential method for solving this problem might be to control the RH of the sample entering the sensor. Sample treatment to remove or stabilise humidity levels is well known in large fixed gas detection installations (eg. via cold finger traps or drying filters), albeit at the expense of the rapid response times which are generally required in portable or real-time analysis applications. However, there is currently no viable, low cost, maintenance-free way of achieving the same result in personal monitors.
Another potential approach might be to measure the ambient humidity with a separate sensor and apply an appropriate correction factor. However, experience shows that the magnitude of the humidity transient varies significantly between sensors having a nominally identical construction and is critically dependent upon the rate of change of RH. It is also likely that the effects vary significantly with sensor age. Thus, a simple solution applicable across a wide range of devices throughout their operating life is unlikely to be achieved in this way.
Yet another potential approach might be to mitigate the RH effect at source, ie. by designing an electrochemical reaction scheme which has no humidity response. However, most gases show high activity on comparatively few catalysts, and so the practical options are quite limited. This often means that a compromise must be made in respect of other undesirable effects, such as cross-sensitivity. Traditionally, the intention has been that the response to the primary, or target, measurand be enhanced whilst leaving the response interferent unaffected, or the reactivity to the interferent reduced with a lesser effect upon the primary or target's sensitivity. Ideally, one seeks to achieve both simultaneously to produce optimum performance.
Methods to offset the impact of cross-interferences, in the form of chemical (gas) contaminants, in electrochemical gas sensors are described in U.S. Pat. No. 4,587,003 and WO-A-2005015195. These approaches rely on either (a) the transmission of unreacted gas through the sensing electrode to a second, compensating electrode; or (b) the provision of parallel gas paths to different electrodes. The signals obtained from the two electrodes are then processed to identify and remove the degree of cross-interference observed. Whilst both methods work adequately for steady-state interferences, they are relatively complex to implement, requiring additional components and changes to the usual sensor hardware. They also tend to be less effective in cases where transient behaviour is involved, due to the difficulty in matching the speed of response of the two electrodes.
Another approach has been to alloy the solid catalyst from which the sensing electrode is made with another metal which reduces the sensitivity of the sensing electrode to the cross-interferent. One example has been the use of a platinum/ruthenium alloy in a sensor for sensing H2S, the ruthenium acting to depress the sensitivity of the platinum to contaminant CO. An alternative is a gold/ruthenium alloy in a sensor for sensing SO2, the ruthenium acting to depress the sensitivity of the gold to contaminant CO.
Chemical filters may also be used to improve the selectivity of sensors, a common example being the use of carbon cloth to remove H2S and alcohols from a CO sensor.
Bias voltage may also be used to remove sensitivity of a sensor to cross-interferences.